Method of fixing images obtained by liquid development in electrophotography

ABSTRACT

A METHOD OF PERMANENTLY FIXING A TONER IMAGE ON AN ELECTROPHOTOGRAPHIC SENSITIVE MATERIAL, SAID TONER IMAGE BEING FORMED BY DEVELOPING AN ELECTROSTATIC LATENT IMAGE PROVIDED ON THE PHOTOCONDUCTIVE COATING OF SAID MATERIAL WITH A LIQUID DEVELOPER COMPRISING A CHARGED TONER DISPERSED IN A VOLATILE INSULATING CARRIER LIQUID, COMPRISING LAMINATING A TRANSPARENT PLASTIC FILM BY MEANS OF AN ADHESIVE LAYER PROVIDED ON SAID FILM ON SAID TONER-IMAGE BEARING SURFACE BY HEAT AND/OR PRESSURE WHILE SAID MATERIAL IS WET WITH THE VOLATILE LIQUID USED IN THE TREATMENT   OF SAID MATERIAL TO ASSIST THE LAMINATION OF SAID FILM ONTO SAID TONER-IMAGE BEARING SURFACE.

Jan. 29. 1974 SATORU HOMO ET AL 3,788,930

METHOD OF FIXING IMAGES OBTAINED BY LIQUID DEVELOPMENT INELECTROPHOTOGRAPHY Filed Oct. 29, 1971 FIG. I

United States Patent O US. Cl. 156-307 4 Claims ABSTRACT OF THEDISCLOSURE A method of permanently fixing a toner image on anelectrophotographic sensitive material, said toner image being formed bydeveloping an electrostatic latent image provided on the photoconductivecoating of said material with a liquid developer comprising a chargedtoner dispersed in a volatile insulating carrier-liquid, comprisinglaminating a transparent plastic film by means of an adhesive layerprovided on said film on said toner-image bearing surface by heat and/orpressure while said material is wet with the volatile liquid used in thetreatment of said material to assist the lamination of said film ontosaid toner-image bearing surface.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinalcross-sectional view of an electrophotographic sensitive material usedin practicing the fixing method according to the present invention.

FIG. 2 is a pictorial view illustrating the fixing method according tothe present invention.

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a methodfor realizing quick and perfect fixing of toner images by forming atransparent surface layer on the image-bearing surface which has beentreated with a liquid developer in electrophotography.

Electrophotography is already widely utilized in document copying forbusiness use. In the case of dry development, an image formed bydepositing dry powder toner is softened and fixed by applying heat or asolvent, while in the case of liquid development, a toner imagedeposited from the carrier liquid of a developer has the self-fixingproperty, so the image is firmly bonded onto the photosensitive layer bya resin ingredient included in the image after the evaporation of thecarrier liquid.

In general, a photosensitive layer composed of a mixture of an inorganicphotoconductor such as zinc oxide and a resin binder has a high volumecontent of powder and, hence, the surface is rough lacking in glossyappearance. However, when a toner image is formed and fixed thereon, theimage portion comes to assume luster. That is, the background portionremains mat while the image portion becomes glossy. Such appearance,however, is not only far inferior to the uniform appearance of aconventional glossy silver-halide print, but also has the disadvantagesthat the background portion is likely to be damaged during storage andthat the toner image may deteriorate when subjected to a strong externalforce. Particularly when the developed image has tonal gradation With awide area of high concentration with a maxi- 3,788,930 Patented Jan. 29,1974 mum optical reflective density as high as about 2.0, it takes along time until the image portion is perfectly fixed and assumes asatisfactorily high mechanical strength. Accordingly, such image islikely to get damaged.

Therefore, the primary object of the present invention is to provide animproved fixing method which is free of the defects inherent to theconventional methods and to obtain an electrophotographic print whichhas an improved appearance and durability well comparable with those ofthe silver-halide prints.

According to the method of the present invention, the electrostaticlatent images are formed on an electrophotographic light sensitivematerial having a support which is preferably flexible, then they aredeveloped with a liquid developer and thereafter a transparent plasticfilm having an adhesive layer is laminated on the photoconductivecoating while the material is still wet with the remaining developerthereby to let said film firmly adhere to the sensitive layer.

The material may preferably be washed with a washing liquid afterdevelopment and prior to fixing operation. Such washing step iseifective to remove the excessive developer liquid present on thephotoconductive coating, and thus to reduce background in the finalprint.

When the material is washed, the lamination is carried out while thematerial is still wet with such washing liquid. Anyway, the presentprocess is characterized by that the lamination fixing is carried out onthe coating which is still wet with a volatile liquid.

Now, the invention will be described in detail with reference to theaccompanying drawings.

Referring to 'FIG. 1, there is shown a sectional view of anelectrophotographic light-sensitive material, generally designated by 1.This sensitive material 1 consists of a support 10, an electroconductivesubcoating layer 11, a back coating 13 and a photoconductive coating 12.The photosensitive material suitably used in the fixing method of thepresent invention may be free of the intermediate layers 11 and 13. Themost preferred material for composing the support 10 is paper because ofits air permeability, appearance and hand. It is also pos sible to usenon-woven fabrics, various types of cloth or so-called synthetic paper.

According to the present invention, an electrophotographic material suchas mentioned above is subjected to exposure and development treatment inthe manner known in the art, and then a transparent sheet is laminatedon said material while the latter is still wet with the volatile liquidwhich has been applied on the material. It was found against ourintuitive expectation that no air bubbles are formed between thelaminated sheet and the electrophotographic coating even if the supporthas no air permeability. In some typical examples, the lamination wasconducted on an electrophotographic material with a paper supportprovided at its back side with a solvent penetration preventive layer,and an aluminumdeposited polyethylene terephthalate film. Thephotoconductive coating was a porous ZnO layer and wetted with anisoparaffinic solvent. Generally, a ZnO coating absorbs such organicliquid of around 10% of the coating weight. This liquid, however,produced no bubbles during the laminating operation, and the operationcould be performed in a satisfactory manner even on a polyethyleneterephthalate film. In the case of a paper with about 4 total thicknessand provided with a back, solvent barrier coating, the sheet absorbed anorganic liquid in an amount equivalent to 7-10% weight of the sheet,still lamination could be achieved satisfactorily without beingaccompanied by blistering. Air permeability of this paper, when measuredby a Gurley-Hill testing machine, had an apparent value of more than 600seconds. This value probably seems to mean the air leakage rate alongthe specimen surface.

These advantageous results are considered attributable to the fact thatthe volatile liquid was retained within the support in the case of theliquid-absorptive support. In the case of the non-absorptive supportsuch as polyethylene terephthalate film, the successful result issupposedly ascribed to the limited amount of liquid retained on thematerial. At any rate, it seems important that the temperature at whichthe lamination is carried out should be kept substantially lower thanthe boiling point of the liquid remaining on the material.

In order to obtain a high quality image of continuous tone on a papersupport, the surface thereof should prefererably be smooth. For thisreason, it is desirable to pro vide a clay layer below the sensitivelayer 12 (see FIG. 1). It is particularly desirable to use art paper,machine coated paper, photographic baryta paper or the like as base andprovide an electroconductive layer 11 on such coated layer. Theelectroconductive layer 11 may be composed of colloidal alumina,polyvinyl benzene sodium sulfonate, polyvinyl benzyl trimethyl ammoniumchloride, sodium or potassium polyacrylate or the like. The back coating13 should preferably be one which can prevent penetration of thedeveloper or pretreating liqiud which may be applied prior todevelopment. The back coating also works to minimize curling of thesensitive materials as a whole. A higher electroconductivity is alsodesired also for the back coating. All of the above-mentioned materialsfor the electroconductive layer can be utilized. Also, CMC, gelatin,starch, casein, polyvinylalcohol or the like may be blended, if desired.

The photosensitive layer 12 should preferably be of a porous structurewhich allows impregnation of an organic solvent to some extent. Atypical example is a homogeneous mixture of inorganic photoconductivepowder such as zinc oxide, zinc sulfide, titanium oxide, cadmium sulfideor the like and binder resin. Organic photoconductive layers havingporous structure may also be used. A photosensitive layer of a typecomposed of photoconductive powder and binder resin may have poorsolvent permeability if the content of the binder is high. It istherefore desirable that particulate component is contained in a highconcentration the preferred range of such concentration being from about30 to 80% of the coating in volume. In the case of zinc oxide, thepreferred mixing rate is 3:1 to :1 by weight ratio. In case of using anorganic photoconductive layer, it is possible to easily obtain a desiredporous structure by suitably adjusting the coating and dryingconditions. It is also possible to additionally disperse powderuniformly therein. For example, a mixture of a good solvent having a lowboiling point and a poor solvent having a higher boiling point may beused as the coating solvent for obtaining a desired porous structure. Ifsuch coating mixture is applied to the support and dried, the good"solvent first evaporates, allowing the poor solvent alone to remain, sothat the film-forming component dissolved in the mixture finallyseparates out at a later stage of drying. The resultantly formed filmhas desired porosity and light-scattering property.

The electrostatic latent images are then formed on such photoconductivelayer. A typical method for forming such electrostatic latent imagescomprises uniform charging of the layer in a dark place and imageexposure. Other known methods can of course be utilized. For instance,Karmans method, where charging and exposure are conducted in thereversed order, may be employed. Also, only the light image exposure maybe effected, without con- 4 ducting electrostatic charging, whereby alatent image is formed by the photovoltaic effect. The latent imagesthusformed are converted to visible images with a liquid developerprepared by dispersing fine charged particles in an insulating carrierliquid. When the high quality images, especially having tonal gradationis required, the latent image bearing surface may be wetted with aninsulating toner-free pre-bath liquid prior to the development. Thistreatment with the pre-bath liquid has the effect to prevent undesirablebackground which may otherwise be caused by trapping of the toners onthe latent image bearing surface by other than the electrostaticfactors. Further, such pre-bath liquid delays attenuation of the latentimages to elongate their file, time besides, penetrates into the backside of the sensitive material to prevent instrusion of the developer,thus also reducing the boiling of the back surfaces of the material.This operation, therefore, is particularly useful for a material havinga back side which is absorptive of the treating liquid used. Washing ofthe developed material is also effective to improve the quality of theprint. Particularly useful washing method is to use a liquid which actsto deposit resinous ingredients contained in the carrier liquid of thedeveloper used, since the image is imparted with mechanical strength.(See German Patent Publication 1,908,298.) The washing liquid usedshould preferably be one which meets the above-said conditions and alsohas a faster evaporation rate than the developer. For practical use, anisoparaffin solvent having a low boiling point, such as, for exampleIsoper E (trade name) manufactured by Humble Oil Co., is preferred. Incase where infiammability should be evaded, chloro-fluorinatedhydrocarbons maybe used either solely or in combination with othersolvents.

The material thus developed and washed is fulfilled mainly with thepre-bath liquid in the bulk (while a thin layer of washing liquid stayson the surface). Such material, therefore, is preferably squeezedlightly with rollers to reduce the impregnated amount of liquid so as topromote drying of the material. In this case, however, a due care shouldbe given since the images are often damaged during the squeezingoperation.

One of the advantages brought about by the present invention is that itmitigates the squeezing condition considerably, because the subsequentlaminating operation can be successfully performed on the wet material.

The photosensitive material, which was squeezed in the manner describedabove and which is still wet, is then supplied to a fixing station suchas shown in FIG. 2. In the figure, reference numeral 1 denotes adeveloped sheet, 10 a support, 12 a sensitive layer and 14 the tonerimages, respectively. A laminate film 2 is delivered from a stock rollnot shown. This laminate film is composed of a transparent base 20 and aheat-sensitive or pressure-sensitive adhesive layer 21. Also in thefigure, numeral 33 designates a tension roll, 31 a heating and pressureroll, and 32 a back-up roll. The heating-pressure roll 31 is made ofmetal and the back-up roll 32 is preferably provided with elasticsurface comprising, for example, Teflon (polytetrafluoroethylene) orsilicon rubber.

In case the adhesive layer 21 is heat-sensitive, the roll 31 should beone which can be heated to a suitable temperature, while in case saidlayer is pressure-sensitive, said roll may be an ordinary pressure roll.It is desirable that the adhesive layer 21 contains a material which issoluble or swellable in the liquid retained in the sheet.

It was unexpectedly found that when the laminate film 2 is pressedagainst the surface of the photosensitive material 1, the remainingliquid on said sensitive material surface or the presence of thevolatile liquid in the photosensitive layer having a porous structure ofsaid sensitive material 1 does not impede a firm bonding. It was alsoconfirmed that any inclusion of air bubbles, which was feared to becaused by air permeability of the support 10, does not actually takeplace.

These results of experiments showed that it is possible to incorporatethe laminate fixing mechanism in the elec trophotographic treatingapparatus to treat the photosensitive material while it is still wetimmediately after development treatment, with no need of perfectlydrying the photosensitive material.

The laminate film used in the present invention may be prepared byapplying a suitable bonding agent to a plastic film having a thicknessof several ten to several hundred ,u. and made of, for example,polyethylene, polypropylene, ethylenevinylacetate copolymer, polyvinylchloride, polyvinylidene chloride, cellulose diacetate, cellulosetriacetate, cellulose acetate, polyethylene terephthalate, polycarbonateor the like.

The pretreatment of the plastic base films which is conducted beforecoating of the adhesive layer includes, for instance, corona discharge,flame treatment, chemical oxidation, ultraviolet ray irradiation orsuitable undercoating. The composition of the adhesive layer isdetermined in association with the base material, and, hence, no furtherdescription may be given here. However, in case of the heat sensitiveadhesive layer, usually a composition mainly comprising a thermoplasticpolymer having a suitable softening point (around 100 C. or less) isused. For further information refer to pages 447 to 451 of 'Handbook ofAdhesives by Irving Skeist (published by Reinhold Publishing Co. in1962).

As mentioned before, it is desirable that the adhesive layer contains amaterial which is soluble or swellable in the insulating liquidremaining in the photosensitive material. Such insulating liquidsinclude usually kerosene chlorofluorinated hydrocarbon, isoparaffin orn-parafiin type hydrocarbon, dimethylpolysiloxane or the like. Among theresin materials which are soluble in such solvents and may beincorporated in the adhesive layer include: polyisobutylene,polyoctylmethacrylate, polylaurylmethacrylate, polybutylmethacrylate,polybutene, petroleum resin, coumarone-indene resin, aliphaticpolyester, long-oil type alkyd resin, polystyrene, phenolformaldehyderesin, glycerine ester of hydrogenated rosin, s-tyrene/ isobutylenecopolyrner, styrene/butadiene copolymer, styrene/higher alkylmethacrylate copolymer and the like. Some of these materials showadhesive property at normal temperature, but for practical use, thosewhich are nonadhesive at normal temperature are found convenient forhandling. It is essential that the adhesive layer show discoloration fora long time. In this respect, one of the most preferred resin materialsis polyalkylmethacrylate in which the alkyl contains carbon atoms morethan 4.

It was found that even if the adhesive layer contains no componentsoluble in the remaining liquid in the photosensitive sheet, firmbonding is obtained as long as the composition of the photosensitivelayer is mainly comprised of thermoplastic ingredients which aresoftened at the temperature where the bonding is practiced. And if thesoluble components are present in the adhesive layer, it is possible toobtain strong bonding even if the photosensitive layer is not softened.

Generally speaking, the presence of such ingredient that is soluble orswellable in the remaining liquid in or on the electrophotographicmaterial favors to realize a firm bonding between the laminate film andthe material. This is also true in the case where theelectrophotographic coating is thermoplastic. Usually, the laminatingoperation is performed at a temperature from room temperature to about120 C. Too low a temperature weakens the bonding, while too high atemperature may cause thermal contraction of the laminate film and, insome cases, may give rise to air bubbles. Therefore in consideration ofthe bonding force and drying speed, the most preferred temperature rangeis 50 to 60 C. Particularly good result is obtained in case the adhesivelayer is mainly composed of a thermoplastic resin having a secondarytransition point within the range of about 15 to about 35 0., because,in such case, no lateral shear is produced at normal temperature andalso no displacement of the laminate film of the obtained copy takesplace during handling at normal temperature.

In the method of the present invention a state that anelectrophotographic material is still wet a liquid implies that thematerial maintains the liquid in the range of about 4 to 30 g. persquare meter. In a photosensitive zinc oxide paper with a solventbarrier backing, when it is subjected to a strong roller squeezing, thecontent of liquid e.g. isoparaffin (s.g. =ca. 0.8) may be reduced to 6to 8 g. per square meter.

Example 1 An art paper measuring 130 g./m. which is available on themarket, was coated on its coated side with 1.0 g./m. (measured in driedstate) of colloidal alumina and on its back side with 1.5 g./m. of thesame alumina. A mixture solvent consisting of methanol and water (70:30)was used in this back coating.

The air permeability of the paper thus obtained was more than 600seconds as measured by Gurley-Hill testing machine.

Then a homogeneous dispersion liquid containing 100 parts ofphotoconductive zinc oxide, 10 parts of styrenemodified alkyd resin and4 parts of polyisocyanate compound was applied on the front side of thepaper which was treated as described above, such that the dry weight ofthis coating will measure 25 g./m.

After thorough drying the photosensitive paper obtained in this mannerwas subjected to dark adaptation, then charged with a negative coronaand exposed to a light image to form an electrostatic latent image. Thissheet was prebathed with kerosene and then developed by using adeveloper which was prepared by mixing carbon black and soybean alkydresin as charge controlling agent in an isoparafiinic solvent. Adevelopment electrode was used during development. Upon completion ofthe development, the sheet was washed with the same isoparaflinic typesolvent (trade name Isoper -E manufactured by *Esso or Humble Oil Co.)to remove the extra developing liquid. Finally, the sheet was lightlysqueezed with squeezing rollers and then immediately passed between aheated roller (heated to C.) and a back-up roller together with alaminate film to be described later. The passing velocity was 5 cm./sec.and the diameter of the heated roller was 4 cm. The film was extendedout so that it contacts with said roller at an angle of about 90. Thelaminate film used here was one which was prepared by exposingpolypropylene film to corona discharge and then immediately appliedthereon with a toluene solution composed of 80 parts ofpolybutylmethacrylate and 20 parts of aliphatic polyester resin (tradename D-tite 307NH manufactured by Dainippon Ink Co. and sold by JapanReichhold Chemical Co.).

The laminate film adhered firmly to the photosensitive layer withoutproducing any blister or peeling-01f. On another developer sheet thesame adhesive film was laminated by reducing the heated rollertemperature to 40 C. and 20 C., respectively; the resulting printconfirmed strong adhesion and uniform appearance.

The photosensitive paper before lamination held 22 g./m. of solvent.Polybutyl methacrylate in the adhesive layer is soluble to theisoparafiinic solvent and kerosene, however, D-tite 307NH is insolubleto them.

'Example 2 A mixture comprising 20 parts of copolymer of styrene, butylmethacrylate and acrylic acid copolymerized in the ratio of 50:45 :5 andparts of photoconductive zinc oxide was added with a small amount ofsensitizing dyes (a mixture of Fluorescein, Rose Bengal andtetrabromophenol blue) to prepare a coating mixture which was coated onthe coated side of g./m. weight baryta paper treated as follows. Thebaryta surface was subbed with 2g./m. of 'ECR 34 (polycationic 1conductive resin made by Dow Chemical Co.) and the back surface wascoated with a thin layer comprising 60% polyvinylbenzene potassiumsulfonate and 40% polyvinyl alcohol to give a dried coating weight of1.5 g./m. No solvent permeation from the back side of this paper tookplace.

An electrostatic latent image was formed on the photosensitive layer ofthis photosensitive material and was developed by using a carbon blacktype developer where resin-modified phenol formaldehyde resin was addedas charge controlling agent in a kerosene carrier liquid. Afterdevelopment the sheet was rinsed with an isoparaffin type solvent(Isoper E produced by Esso or Humble Oil 00.). After rinsing, the sheetwas held vertically for a short time to let most of the liquid flowdown. Under this condition, the sheet retained about g./m. of theliquid.

Then a polyvinyl chloride film having an adhesive layer composed of amixture of 70' parts of polyvinyl acetate and parts of coumaron-indeneresin was laminated on said already developed wet photosensitivematerial. The obtained copy had solid film adhesion and no air bubblewas observed.

Example 3 A laminate film having an adhesive layer composed of 40 partsof glycerin ester of hydrogenated resin and 60 parts of D-tite 307N'H(trade name) was applied on an already developed photosensitive materialobtained in the same manner as in Example 1. The result was excellent.

Example 4 A colored image was reproduced on a photosensitive paper ofExample 1 according to an over-printing method using developers of threedifferent colors. The carrier liquid was a mixture of 75 parts ofdecalin and 25 parts of kerosene. A varnish prepared by dissolvingresinmodified phenol resin in linseed oil under heat was used as chargecontrolling agent. Rinsing was carried out with a mixture solution ofIsoper and Diflon S-2 (CCI F-CCl F, produced by Daikin Industries). Alaminate film formed by providing an adhesive layer composed of D-tite307NH alone on a polypropylene film was pressed against the wetphotosensitive sheet. There was resultantly obtained a duplicate havingfirm and uniform adhesion of said laminate film.

1 Polyvinylbenztrimethylammonlum chloride.

What is claimed is:

1. A method of permanently fixing a toner image on anelectrophotographic sensitive material, said toner image being formed bydeveloping an electrostatic latent image provided on the photoconductivecoating of said material with a liquid developer comprising a chargedtoner dispersed in a volatile insulating carrier liquid, comprisinglaminating a transparent plastic film by means of an adhesive layerprovided on said film on said toner-image bearing surface by heat and/orby pressure while said material is wet with the volatile liquid used inthe treatment of said material to assist the lamination of said filmonto said toner-image bearing surface.

2. A method of permanently fixing a toner image on anelectrophotographic sensitive material, said toner image being formed bydeveloping an electrostatic latent image provided on the photoconductivecoating of said material with a liquid developer comprising a chargedtoner dispersed in a volatile insulating carrier liquid, comprisinglaminating a transparent plastic film by means of an adhesive layerprovided on said film on said toner-image bearing surface by heat and/orby pressure while said material is wet with the volatile liquid used inthe treatment of said material, the adhesive layer of the filmcontaining an ingredient which is soluble or strongly swollen by thevolatile liquid remaining on/in said material.

3. A method of permanently fixing a toner image on anelectrophotographic sensitive material, said toner image being formed bydeveloping an electrostatic latent image provided on the photoconductivecoating of said material with a liquid developer comprising a chargedtoner dispersed in a volatile insulating carrier liquid, comprisinglaminating a transparent plastic film by means of an adhesive layerprovided on said film on said toner-image bearing surface by heat and/orby pressure while said material is wet with the volatile liquid used inthe treatment of said material, the amount of volatile liquid held bythe sheet material just before the laminating operation ranging between4 g./m. to 30 g./m.

4. A method of permanently fixing a toner image corresponding to acontinuous tone original, said toner image being on anelectrophotographic sensitive material and formed by developing anelectrostatic latent image provided on the photoconductive coating ofsaid material with a liquid developer comprising a charged tonerdispersed in a volatile insulating carrier liquid, comprising laminatinga transparent plastic film by means of an adhesive layer provided onsaid film on said toner-image bearing surface by heat and/ or bypressure while said material is wet with the volatile liquid used in thetreatment of said material.

References Cited UNITED STATES PATENTS 3,669,859 6/1972 Merrill 961 R X3,549,447 12/1970 Bresnick 961 R UX 3,427,242 2/ 1969 Mihajlov 96l.3 X

RALPH S. KENDALL, Primary Examiner D. A. SIMMONS, Assistant Examiner US.Cl. X.R.

